1. Field of the Invention
The present invention relates generally to subsurface equipment for wellbores and, more particularly, to subsurface equipment used to create separate annuli for production and working fluids.
2. Description of the Related Art
Wellbores are often provided with separate, multiple flow channels for moving fluids into and out of subsurface reservoirs. For example, a single injection well may be required to provide injection fluids to two or more layers in a reservoir, in which case two or more separate flow channels are required. As another example, a single wellbore may be used to provide both a means for producing fluid from a reservoir and also for providing a supply and return conduit for supplying a working fluid to a subsurface device.
One way of separating the flow channels is to use separate tubing strings in parallel and placed into a single wellbore. This method is useful for shallow wells having low flow rates but is impractical for wells having higher flow rates or deep wells where pressure drops caused by the required narrow tubing strings are unacceptable. Instead, concentric tubing strings are used, wherein one or more tubing strings are nested one inside another creating multiple annular flow channels defined by the inner wall of a first tubing string and the outer wall of a second tubing string passing through the annulus of the first tubing string. As the annular flow channels are separated by the tubing walls, the annular flow channels are isolated from one another in regard to pressure and the exchange of fluids. In addition, insulated tubing strings may also provide some thermal isolation between the annular flow channels.
One problem associated with concentric tubing strings is that the assignment of the fluids in each annular fluid channel is typically fixed. That is, once a fluid enters one of the annular flow channels, it must remain in that annular fluid channel and cannot be switched with fluid from another annular fluid channel. This may cause a problem, for example, when a subsurface device, such as turbine driven pump, needs to be placed in the wellbore and fluid needs to be routed to the device around another intervening device in the tubing string.